The invention relates to a voltage controlled equalization circuit which is particularly suitable for equalization of signals transmitted over a channel having a non-uniform frequency response, including signals subjected to magnetic recording and reproduction processes.
A basic requirement for ideal signal transmission channel is the ability to transmit signals over a range of frequencies without introducing amplitude or phase changes dependent on frequency to avoid distortion. However, such transmitted signals are generally distorted due to a non-constant amplitude response and non-linear phase response inherent to the transmission process. Consequently, to obtain an accurate replica of the original signal upon reception or reproduction, it is necessary to compensate for the signal distortion.
A large variety of prior art amplitude equalizers are known which compensate for amplitude distortion of this type. These equalizers generally include circuits having an amplitude versus frequency response characteristic which compensates for the amplitude losses caused by the non-uniform frequency response of the channel. It is a general requirement that these amplitude equalizers do not introduce additional phase shift, and thereby distortion, into the equalized signal. To accurately match the amplitude versus frequency characteristic of the equalizer to that of the transmission channel, it is necessary to provide equalizers with adjustable response. Normally such amplitude equalizers have a gain response which rises with frequency. The amount of rise is called boost.
Delay line amplitude equalizers are known to utilize a delay line coupled between two inputs of a differential amplifier. More specifically, a delay line is coupled between an inverting and a non-inverting input of a differential amplifier. The differential amplifier operates as a difference circuit which provides an output signal corresponding to a difference between its input signals. The resulting circuit provides the desired equalization signal.
Known equalizers utilize manually operated potentiometers for the frequency response adjustment. Such potentiometers have an adjustable output, also known as a wiper contact, connected in the main equalization signal path. A minimum boost of the equalization signal is obtained when the adjustable wiper is positioned near one terminal while a maximum boost is obtained when the adjustable wiper is moved to the other, opposite terminal of the potentiometer. To adjust that equalizer boost it is necessary to mechanically adjust the position of the wiper contact of the potentiometer. The aforementioned electromechanical boost control is satisfactory for manually controlled equalizers having controls located close to the device. However, when the equalizer is at a location distant from the control station, remotely controlled mechanical means are necessary to obtain a desired adjustment.
Also, in applications utilizing automatic boost adjustment, for example by a computer generated electrical output signal, use of potentiometers would require additional devices for converting the electrical signal into a mechanical control signal.
Replacing the potentiometer for example by a remotely controllable voltage controlled amplifier introduces an undesirable delay into the equalization signal path at high frequencies and thus an additional phase shift, thereby upsetting the equalization process. Even when additional compensation networks are added to compensate for the delay, the quality of equalization is impaired in comparison with devices utilizing potentiometer control.